The following invention relates in general to a pH-indicator composition consisting of two components, which together show an elevated freezing point and effect a color change at selected freezing temperatures, and to a method of making such a composition. More specifically, the invention aims to utilize such color change, as per the chosen methods, to obtain new compositions for temperature indication, especially down to xe2x88x9225xc2x0 C. or lower. The field of application for the invention is many, for example, different temperature indicators that can guard temperature-sensitive cold storage or frozen products.
A number of materials, such as finished solutions of pharmaceuticals, vaccines, blood products, chemical products, food products, flowers, etc., can be damaged upon freezing. According to the present invention, a freeze-warning composition can give an early visual warning by color change. In this way, one may easily take steps for corrective action before freezing damages them.
It is generally known that ice packs are used to maintain the cold temperature of cold- storage products during transportation. Usually, ice packs are stored in freezers at xe2x88x9218xc2x0 C. or lower and placed together with temperature-sensitive cold storage products, such as vaccines or other pharmaceuticals. Sometimes, freeze-sensitive cold storage products are affected because the ice packs have been too cold. In a WHO investigation (WHO/EPI/CCIS/83.7) concerning vaccine transportation, it was observed that between 7.4 and 10.6 per cent of vaccines were damaged by freezing. The WHO recommended that ice packs, after being taken out from the freezer, should be stored at room temperature for sometime before packing together with vaccines. The intention was to raise the temperature of the ice packs to around xe2x88x925xc2x0 C. so those freeze-sensitive products were not exposed to freezing risk.
Unnecessary energy is spent to freeze the liquid in the ice packs to xe2x88x9218xc2x0 C. or lower and afterwards warming them up to between xe2x88x924xc2x0 C. or xe2x88x925xc2x0 C. before usage. The xe2x88x924xc2x0 C. indicators (based on U.S. Pat. No. 4,147,852) which the WHO tested, did not freeze or change color until they were initially exposed to a temperature of xe2x88x9210xc2x0 C. or lower. Here, our invention can be applied.
The problem has been that temperature indicators that contain salt solutions and are enclosed in sealed glass tubes or plastic packets do not freeze readily. Not before the temperature is lowered too far below the freezing point does a change occur. In addition, the contents in these packages freeze irregularly and with broad temperature intervals, often between xe2x88x926xc2x0 C. and xe2x88x9212xc2x0 C. WHO:s investigation (WHO Weekly Epid. Rec. Nos.50 and 51, 1980) did show that vaccine preparations freeze under a wide interval between xe2x88x925xc2x0 C. and xe2x88x9210xc2x0 C. To solve the problem, a composition according to the present invention can be advantageously applied. This can give an early visual indication that risk for freezing is impending. In principle, pH-indicators represent acid-base character, where the acid side has another color than the basic. One has used this property in acid-base titrations and to prepare temperature dependent indicators based on color change.
U.S. Pat. No. 2,261,473 describes that certain organic or inorganic substances change their acid-base character upon melting or solidification as a result of intramolecular rearrangement. These changes can influence the color of pH indicators, which change reversibly from acid to base color or vice versa.
U.S. Pat. No. 2,195,395 describes a method to show color changes of pH-indicators irreversibly. To the composition is added ester compounds, which at selected temperatures, decompose to acid and alcohol. This is shown by color change.
U.S. Pat. No. 4,147,852 describes a method for regulating color changes of pH-indicators in an aqueous solution at temperatures far below the freezing point. This is made possible by adding certain regulating agents. In the presence of these regulating agents, the composition maintains its color without changing upon freezing. Color change occurs far below the freezing point depending upon the concentration of the added regulatory agent. In such a manner an extended temperature interval between freezing and color change is made possible. In the same patent document are also described results of certain experiments without the addition of regulating agents. Color change was observed upon exposition to between xe2x88x9218xc2x0 C. and xe2x88x9220xc2x0 C., i.e. far below the compositions"" freezing points. Without having tested if color change takes place at the freezing point itself, one concluded that color change occurred in association with or solely upon freezing (solidification) in analogy with the description in U.S. Pat. No. 2,261,473.
We have in the present invention established that color change does not occur when buffer solutions containing pH-indicators freeze. The pH-values of the buffer solutions, molarity and ionic strength influence the temperature interval between the freezing point and change of color. Therefore, it is not obvious that color change occurs solely upon freezing.
Accordingly, this invention provides a composition comprising two components. Which together show an elevated freezing point and bring about color change at selected freeze temperatures, wherein the composition comprises as Component 1 a buffer solution containing at least one pH indicator, and as Component 2 at least a solid material in the form of a metal object, which hastens freezing and color change.
In another embodiment of the invention, a composition of the invention comprises two components, which together show an elevated freezing point of a salt solution and brings about a color change at selected temperatures below the freezing point in a reversible way. The composition comprises a first component of a buffer salt solution containing at least one pH indicator, and as a second component, at least one water insoluble solid material that hastens freezing.
In a further embodiment the invention provides a composition comprising two components, which together show an elevated freezing point of a salt solution and shows, by color change, when a selected temperature below freezing is above or below this level. The composition comprises a first component of a buffer salt solution containing at least one pH indicator, and as a second component, at least one water insoluble solid material that hastens freezing.
In a preferred embodiment of the invention, the insoluble material that hastens freezing is a metal ball.